Understanding Overload Failures

Let's face it: things break. In every manufacturing company, chances are good that at some time or another, you'll find a group of engineers and technicians standing around a broken part, scratching their heads, and asking, "Why?"

Sometimes parts break during testing under extreme conditions. These failures help us to understand the limits of a design. Other times, parts fail unexpectedly in service. These are the failures we would all like to avoid. At best, they lead to unhappy customers. At worst, they can lead to people being hurt or even killed. The good news is that a solid understanding of potential failure modes can help us prevent service failures.

The simplest failure mode to understand is overload. This is what happens when the load on a part exceeds the material's strength. Exceeding the yield strength will cause the part to permanently deform. Exceeding the ultimate strength will cause it to break. In either case, the failure occurs immediately.

Assuming you've done your stress analysis right, parts shouldn't fail in overload under normal operating conditions. So an overload failure means one of two things: either the load was too high, or the material's strength was too low.

As an example of the latter, the cam lever of a mechanical brake bent permanently the first time the brake was applied. The bend reduced the lever's stroke, preventing the brake from fully actuating. Fortunately, all of the brakes were tested prior to being shipped to the customer; otherwise, this could have been a dangerous situation. (This was an emergency brake for heavy equipment.) It turned out that the lever supplier had substituted hot-rolled, low-carbon steel for the cold-rolled, medium-carbon steel which had been specified. The steel the supplier used had a much lower yield strength than the specified steel. All of the levers had to be replaced, at the supplier's expense. The attempt to substitute a cheaper material turned out to be quite costly for the supplier.

Substitution of one material for another is not the only reason why a material's strength might be low. For example, improper heat treatment -- usually too high a temperature, too long of a time, or too slow of a cooling rate -- can lead to low strength. (For carburizing or nitriding heat treatments, too low of a temperature or too short of a time can also lead to low strength.) But even if a part is made from the correct material and receives the correct heat treatment, things can happen in service, which may reduce its strength. For example, exposure to high temperatures will generally decrease the strength of metal parts. For plastic parts, add ultraviolet light and exposure to certain chemicals to the list of things that can reduce strength.

If a part breaks in overload, and the material's strength is not too low (which can often be determined by a simple hardness test), then the load must have been too high. In many cases, examination of the failed part can provide clues as to which direction the load came from, and what type of load -- tension, compression, torsion, or bending -- it was. The question then becomes: Where did the load come from? Answering this question requires careful consideration of the mechanical system as a whole. Often, the broken part may turn out to have been an innocent bystander. For example, misalignment of another part might have produced abnormally high loads.

Avoiding overload failures, then, is a matter of understanding the forces that act on a part and the properties of the material from which it is made. It's also important to understand variables, which may cause either the loads or the material properties to differ from their normal values. As we'll see, these principles are also the key to understanding other failure modes. In my next post, I'll discuss how to apply these principles to fatigue failures.

As mentioned earlier, repeated stresses/cycles on an assembly are a major contributor to fatigue and crack propagation. The biggest contributor to repeated cycles is vibration. Sometimes it's difficult to observe, but even high frequency vibration (though very small displacements) can be a fatigue factor due to their high cycle rate.

Vibration can be an issue when attaching a component to a moving machine (frequency depends on the machine dynamics), when designed to handle siesmic vibrations near fault lines (relatively low frequency <10Hz), or just designing to handle transportation to the end user (between 2-500Hz). The frequency and amplitudes vary, but the main goal is to design components with resonant frequencies well above what the sample will see while in use or transport, and when designing machinery, to avoid stacking resonant frequencies so the components aren't exciting each other's resonant frequencies while in use.

@Mydesign: You're right that loads redistribute to a certain extent as a result of localized yielding, so that a linear FEA which predicts a stress greater than the yield strength in a small region doesn't necessarily indicate failure of the component. This is why designing to "get the red out" of a FEA model, without any insight into the physical situation, can result in overdesign. On the other hand, stresses below the yield strength can lead to fatigue failure if they are repeatedly applied. To get a handle on fatigue, it's important to know how the loads on a part vary over time. This is what I will discuss in the next installment.

Dave, I think the weight is eventually distributed across the area, and then it can bear more weight than concentrate to particular points. I think in most of the industrial wing, the stress tests are doing for a mass areas rather than stress test in cubic/cm sqd.

@Chuck: Other engineers should feel free to weigh in on this, but in my experience, it's most common to design to the yield strength, with an appropriate factor of safety. Doing this should protect you against overload failures, provided that (as I pointed out in the article) the loads are what you think they are, and the yield strength is what you think it is.

Dave: In the kinds of parts that are mentioned here, such as the brake cam, are the parts typically designed in accordance with the yield strength of the material, or is there some "allowable stress" design method that's set forth that is not dependent on yield? If yield is not used as criteria, does it make any difference in terms of failure rates?

@Alex: Usually, a crack which grows over time is due to fatigue, which I'll cover in my next installment. (In plastic parts, cracks which grow over time could also be due to environmental stress cracking, which I've written about before. In metals, there is a phenomenon called stress corrosion cracking, which is analagous to environmental stress cracking; I might write about this later).

Based on what I've seen in my career, fatigue failures are actually far more common than overload failures. However, overload failures are the easiest to understand, which is why I wanted to cover them first.

The mechanics of fatigue are a little more complicated. As I'll discuss, a common mistake is to treat "fatigue strength" as though it's a property like yield strength or ultimate tensile strength. It's not. But the big picture is the same: you need to understand the forces that act on the part, and the properties of the material from which it is made -- as well as all of the variables which might cause either one to vary from its normal value.

Could you do an explanation in a future post of the differences between a part (say, a bracket on a car) failing due to a migrating stress fracture versus a total, quick failure where it just breaks in two? Is that the same stress dynamic in play with different outcomes, or are they different processes entirely?

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